If Americans snacked only occasionally, and in small amounts, this would not present the enormous problem that it does. But because so much money and effort has been invested over decades in engineering and then relentlessly selling these products, the effects are seemingly impossible to unwind. More than 30 years have passed since Robert Lin first tangled with Frito-Lay on the imperative of the company to deal with the formulation of its snacks, but as we sat at his dining-room table, sifting through his records, the feelings of regret still played on his face. In his view, three decades had been lost, time that he and a lot of other smart scientists could have spent searching for ways to ease the addiction to salt, sugar and fat. “I couldn’t do much about it,” he told me. “I feel so sorry for the public.”